It's true, Castiel doesn't have a full understanding of how much his mere presence drives Dean up the wall, nor does he know the full scope of why it's so painful. He has the basics, though, and while that might be enough to keep someone else away, it doesn't work that way with him.

He realizes that he's one big sore spot, but he still wants to help. He doesn't know this Dean, not really, and so his instincts have him falling back on the very baseline of their relationship, which is that he's here to help Dean.

They've gone back and forth on that a lot, with Dean forcing him to realize that he's more than just a trick up the Winchesters' sleeve. But it's easy to fall back on that in this moment.

"I've already spoken to Tom," he says with a sigh. "I know what we have to do, I've cataloged most of the ghosts, I have that under control." Castiel glances down to the hammer in Dean's hand. "But I think I might be able to put this place together more quickly than that hammer of yours."

I'm not a hammer, as you say. But right now that's all he can be.